Problem: Which of these numbers is prime? ${8,\ 31,\ 77,\ 91,\ 99}$
A prime number has exactly two factors: 1 and itself. The factors of 8 are 1, 2, 4, and 8. The factors of 31 are 1 and 31. The factors of 77 are 1, 7, 11, and 77. The factors of 91 are 1, 7, 13, and 91. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 31 is a prime number.